1. Field of the Invention
The invention relates to structural repair of defects in advanced superalloy components. In some embodiments, the invention relates to repair of cracks or other defects in superalloy turbine blades that are used in steam or gas turbines (industrial or aero), by filling the crack with molten superalloy filler, in a manner that does not significantly reduce mechanical structural or material properties of the superalloy blade substrate. The molten filler superalloy solidifies into a cast in place patch of filler superalloy.
2. Description of the Prior Art
“Structural” repair of gas turbine or other superalloy components is commonly recognized as replacing damaged material with matching alloy material and achieving mechanical properties, such as strength, that are close to the original manufacture component specifications (e.g., seventy percent ultimate tensile strength of the original specification). For example, it is preferable to perform structural repairs on turbine blades that have experienced cracks, so that risk of further crack growth is reduced, and the blades are restored to original material structural and dimensional specifications.
Structural repair of nickel and cobalt based superalloy material that is used to manufacture turbine components, such as cast turbine blades, is challenging, due to the metallurgical properties of the finished blade material. For example, a superalloy having more than 6% by weight percentage aggregate aluminum or titanium content, such as CM247 or DS247 alloy, is more susceptible to strain age cracking when subjected to high-temperature welding than a lower aluminum-titanium content X-750 superalloy. The finished turbine blade alloys are typically strengthened during post casting heat treatments, which render them difficult on which to perform subsequent structural repair welding. Currently used welding processes for superalloy structural fabrication or repair generally involve substantial melting of the substrate adjoining the weld preparation, and complete melting of the welding rod or other filler material added. When a blade constructed of such a material is welded with rods of the same or similar alloy, the blade is susceptible to solidification (aka liquation) cracking within and proximate to the weld, and/or strain age (aka reheat) cracking during subsequent heat treatment, processes intended to restore the superalloy original strength and other material properties comparable to a new component.
In comparison to structural repair, “cosmetic” repair of superalloys is recognized as replacing damaged material with unmatching alloy material of lesser structural property specifications, where the localized original structural performance is not needed. For example, cosmetic repair may be used in order to restore the repaired component's original profile geometry. As noted above, it is desirable to perform structural repairs on surface cracks in order to reduce their likelihood of subsequent spreading when the component is returned to service. Conversely, an example of cosmetic repair is for filling surface pits (as opposed to structural cracks) on a turbine blade airfoil in order to restore its original aerodynamic profile, where the material properties of the blade's localized exterior surface is not critical for structural integrity of the entire blade. Cosmetic repair is often achieved by using oxidation resistant weld or braze alloys of lower strength than the blade body superalloy substrate, but having higher ductility and lower application temperature that does not negatively impact the superalloy substrate's material properties.
Given the shortcomings of superalloy structural repair welding, often the only commercially acceptable solution is to scrap damaged turbine blades that require structural repair, because past experience has shown limited success of such structural repairs. Thus repairs have been limited to those that have in the past been proven to be performed successfully by cosmetic welding, employing more ductile welding rod filler materials with reduced structural strength. In circumstances where structural welding repairs are successfully performed, cosmetic welding repairs are often performed on blade portions that do not need structural repairs. This required two different repair modes to be used on the same blade.
Thus, a need exists in the art for a method for performing structural repairs on surfaces of superalloy components, such as turbine blades, so structurally damaging cracks and other surface defects can be repaired.
Another need exists in the art to increase successful rates of structural repairs on surfaces of superalloy components, such as turbine blades, so that damaged blade scrap rates can be reduced.
Yet another need exists in the art for a method for performing structural repairs of superalloy components, such as turbine blades, with proven, repeatable repair techniques and post-repair heat treatment procedures that do not require complex welding.
Another need also exists in the art for a universal method for performing repairs of superalloy components, such as turbine blades that can be used for repair of all blade defects.